Ultrasonic transducers are often used as impulse mode transducers operating over a wide range of frequencies. Since such transducers need to handle wideband frequency signals, wideband design is an important subject. In the prior art, impedance converters have been placed on a face of a piezoelectric layer of an ultrasonic transducer to improve the wideband frequency response of the transducer. One of the important applications of wideband transducers is in medical imaging systems. Economical, reliable and reproducible mass-production processes for transducers for use in medical imaging systems are particularly desirable.
Impedance converters for ultrasonic transducers are known in the art. As is known in the art, an ultrasonic transducer includes a piezoelectric active layer, one or more front matching layers on a front face of the piezoelectric active layer to serve as an impedance converter, and a backing absorber on a rear face of the piezoelectric active layer. A typical piezoelectric material, such as lead zirconate titanate (also known as “PZT”) has high characteristic acoustic impedance, for example, ZPZT=30×106 kg/m2s (Rayl). A typical propagation medium, such as water, has low characteristic acoustic impedance, for example, ZR=1.5×106 Rayl. Because of the difference in characteristic acoustic impedances of these media, acoustic waves in the piezoelectric active layer of an ultrasonic transducer are reflected backward into the piezoelectric active layer at the boundary between the piezoelectric active layer and the transmission medium (the front boundary) and reflected frontward into the piezoelectric active layer at the back boundary (the boundary between the rear face of the piezoelectric active layer and the material to the rear of the piezoelectric active layer). This results in a resonance at a specific frequency in the ultrasonic transducer, as determined by the half wavelength condition of the piezoelectric material.
When such a resonated transducer is driven by a voltage pulse (when acting as a transmitter) or by an acoustic pulse (when acting as a receiver), the signal wave does not decay quickly (a phenomenon known as ringing). This effectively renders such a transducer unsuitable for imaging systems, in which systems short acoustic pulse beams are excited, directionally scanned and reflected back from a target to enable an image of the target to be constructed. A front impedance conversion layer (also known in the art as a matching layer for reducing reflections) is inserted between the front face of the piezoelectric layer and the propagation medium to mitigate creation of resonance due to the difference in the characteristic acoustic impedances of the piezoelectric material and the front propagation medium.
A piezoelectric layer's vibration excites an acoustic wave in the backward direction, i.e., in a direction away from the front face of the piezoelectric layer. A certain amount of reflection from the back boundary towards the front face may be desirable to improve the sensitivity of the ultrasonic transducer. Often a backing absorber layer of acoustic absorber material is attached to the rear face of the piezoelectric layer. If the characteristic acoustic impedance of the backing absorber material effectively matches that of the piezoelectric material, a significant amount of acoustic wave energy passes through the back boundary without reflection and is absorbed by the backing absorber layer. In such a case, the sensitivity of the transducer is lowered and the bandwidth may become excessive for some applications. Therefore, some mismatch between the characteristic acoustic impedance of the piezoelectric material and the backing absorber material is desirable, depending on the required bandwidth and sensitivity.
The characteristic acoustic impedance of the backing absorber material may be selected to obtain a desired performance of the ultrasonic transducer. If a transducer cannot be provided with a backing absorber material of a suitable characteristic acoustic impedance, a back impedance conversion layer may be added between the piezoelectric active layer and the backing absorber layer to provide the necessary overall acoustic impedance at the back boundary of the piezoelectric layer.
A typical acoustic impedance conversion structure may be a layer of uniform thickness, the thickness equal to about one-quarter of the wavelength of a desired operating wavelength of the acoustic transducer. The acoustic impedance conversion layer has a characteristic acoustic impedance (Zm), which is approximately the geometric mean of the characteristic acoustic impedance (Z1) of the propagation medium and the characteristic acoustic impedance (Zp) of the piezoelectric active layer, i.e., Zm=√(Z1·Zp). Since Z1 is small (Z1=ZR=1.5×106 Rayl), and the characteristic acoustic impedance of the piezoelectric layer is relatively high, the characteristic acoustic impedance Zm of the matching layer is selected to be between those of the propagation medium and the piezoelectric layer, i.e., Zp>Zm>Z1.
One problem associated with a conventional ultrasonic acoustic impedance conversion layer (i.e., quarter wavelength layer) is the difficulty in choosing a material to obtain an appropriate characteristic acoustic impedance Zm for both front and back acoustic impedance conversion layers. More specifically, ultrasonic transducers are often required to operate over a wide bandwidth (for example, 40-60% of the center frequency). To obtain satisfactory performance over such a wide bandwidth using bulk PZT as the piezoelectric active layer, a typical acoustic impedance conversion layer structure used comprises a single front matching layer having a characteristic acoustic impedance of Zm=6.7×106 kg/m2s (Rayl).
Another known acoustic impedance conversion structure providing still wider bandwidth uses double matching layers. Here, two quarter wavelength layers having characteristic acoustic impedance of Zm1 and Zm2 are used. In a structure employing double matching layers, the matching layer with characteristic acoustic impedance Zm1 is in contact with the propagation medium, which has a characteristic acoustic impedance Z1; the matching layer with characteristic acoustic impedance Zm2 contacts the surface of the piezoelectric layer. The materials of the matching layers are chosen to satisfy a specific relation such as Zp>Zm2>Zm1>Z1. However, it is quite difficult to obtain appropriate materials for these layers while satisfying the specific designed values of the characteristic acoustic impedances. For example, polyimide has a characteristic acoustic impedance of 3.16×106 Rayl. Polyester has a characteristic acoustic impedance of 3.4×106 Rayl, PVDF: 3.7×106 Rayl, glass: 13.2×106 Rayl, and aluminum: 17.3×106 Rayl. In addition to choosing a material for the front matching layer having a suitable characteristic acoustic impedance, the material should desirably meet other criteria such as process compatibility, ease of mass-production, and material cost. In the prior art, epoxy loaded with high characteristic acoustic impedance material such as glass fiber or silica powder has been used. However, the thickness and uniformity of such a loaded epoxy proves difficult to control.
Another problem associated with the conventional design of ultrasonic transducers arises in array transducers, where a flexible printed circuit layer or board on which multiple conductor traces are formed is disposed to the rear of the array. Each conductor trace is connected to one element of the array. A backing absorber is then attached on the rear face of the flexible printed circuit board. The acoustic performance of the flexible printed circuit negatively affects the performance of the transducer. The polymer layer of a typical flexible printed circuit board has characteristic acoustic impedance of about 3.2×106Rayl, which is too low and renders the structure insufficient to serve as an adequate matching layer.
When a piezoelectric layer is diced to define an array of elongated elements of narrow width, the kerfs or channels between the elements are filled by a filler material (such as epoxy). As a result, the characteristic acoustic impedance of the piezoelectric layer is reduced. In ultrasonic transducers employing such arrays, the properties of suitable acoustic impedance converters are different from the properties of acoustic impedance converters suitable for transducers having solid piezoelectric active layers. The selection of suitable materials for the acoustic impedance converters is also dependent on bandwidth and sensitivity requirements. Adjusting the characteristic acoustic impedance Zm of acoustic impedance converters using available techniques has proven difficult.
In ultrasonic transducers with no backing absorber, or with air or a very low characteristic acoustic impedance material as a backing absorber, strong reflections from the back boundary causes the transducer to operate with a relatively narrow resonance, or results in a strong resonance peak. In such ultrasonic transducers, the fabrication of an appropriate acoustic impedance converter for the front face may require high quality workmanship and custom materials. When an acoustic impedance converter for the front face of the piezoelectric layer is properly designed and fabricated, a broadband and high efficiency transducer can be produced. However, large scale production of such transducers is difficult to attain due at least in part to the need for skilled artisans having high quality workmanship and custom materials to create such acoustic impedance converters.
The concept of a multilayer acoustic impedance converter having a low characteristic acoustic impedance layer arranged closer to a piezoelectric layer and a high characteristic acoustic impedance layer bonded at the outer surface of the low acoustic impedance layer is also known in the art. In the prior art, both layers are less than one quarter of a wavelength thick. The combined structure provides an effective acoustic impedance conversion equivalent to that of a quarter wavelength scheme. U.S. Pat. No. 6,772,490 teaches multilayer acoustic impedance conversion layers with such a combination of lower and higher characteristic acoustic impedance layers. The effective characteristic acoustic impedance of the multilayer impedance converter of the '490 patent is lower than the characteristic acoustic impedance of the radiation or propagation medium for achieving high sensitivity when operating the transducer at the center resonant frequency. While this design is suitable for effective energy transfer at the center frequency of a narrow bandwidth (which is often suitable for continuous wave excitation) this design exhibits a steep drop in performance as the frequency is changed away from the center frequency. Such design is unsuitable for operating the transducer at broader bandwidths required for applications such as pulse excitation and reception.
Another example of a prior art transducer arrangement is provided in Toda, “New Type of Matching Layer for Air-Coupled Ultrasonic Transducers,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 49, no. 7, July 2002, pp 972-979, which describes a basic design principle of a multilayer acoustic impedance converter for reducing reflection at the front of a piezoelectric layer of a transducer with wideband performance. This is an air acoustic wave transducer. Here, a lower characteristic acoustic impedance layer (formed of air) is disposed at a first surface of the piezoelectric layer and is followed by a higher characteristic acoustic impedance layer (formed of a polymer) contacting the propagation medium of air. Each of these layers is thinner than one quarter wavelength and the combination of these two layers functions as a quarter wavelength impedance converter. For an ultrasonic transducer with water or the human body as the propagation medium having broad bandwidth operation as required for pulse excitation and reception, alternative materials and methods of implementing such transducers are desired.